With the still larger capacity and higher speed of an optical communication system, it has become more important to provide an optical device which carries out advanced functions at a low price. Since a lightwave circuit fabricated on a planar substrate is highly versatile in design and is excellent in mass-producibility and stability, it can provide the optical device meeting the requirement, and researches and developments have therefore been made worldwidely.
Examples of prior-art planar lightwave circuits are shown in FIGS. 40 through 42. The planar circuit shown in FIG. 40 is configured of an input optical waveguide 171, and an output optical waveguide 172 which is optically coupled to the input optical waveguide 171 (refer to, for example, Patent Document 1). The planar lightwave circuit shown in FIG. 40 has the output optical waveguide 172 designed in a parabolic shape, thereby to be endowed with the function of adjusting the field distribution of output signal light.
With the design technique, however, only the fundamental mode of input signal light and the second-order mode coupled thereto can be handled, so that a characteristic as a lens for adjusting the field distribution of the signal light has been inferior. Also, there has occurred the problem that the size of the planar lightwave circuit becomes large on account of a configuration which gradually generates the second-order mode.
Besides, there has been known a planar lightwave circuit which is endowed with a spot-size conversion function by a configuration wherein an optical waveguide of taper shape and its connection part with an optical fiber are periodically divided (refer to, for example, Non-patent Document 1).
Since, however, the optical waveguide propagating a light signal is periodically segmented, there is the problem that the reflection of signal light at each segmented surface is inevitable, and the planar lightwave circuit has had the drawback that it cannot be applied to any other use than a spot size converter.
Besides, there has been known a planar lightwave circuit which is endowed with a spot-size conversion function by a configuration wherein the width of an optical waveguide repeats increases and decreases aperiodically (refer to, for example, Non-patent Document 2).
However, the optical waveguide width repeats abrupt increases and decreases along the propagation direction of signal light, and hence, there has been the problem that the fabrication of the planar lightwave circuit is very difficult.
Shown in FIG. 41 is the configuration of a lightwave circuit including a prior-art cross waveguide. The cross waveguide is indispensable as one of basic constituents in the lightwave circuit. The lightwave circuit 260 shown in FIG. 41 includes two input optical waveguides 261, two output optical waveguides 264, and an optical-waveguide crossing portion 265 being a waveguide overlap portion which couples the two input optical waveguides 261 and the two output optical waveguides 264, respectively. The crossing angle 266 between the input optical waveguide 261 and the output optical waveguide 264 needs to be narrowed for attaining reduction in the size of an optical device. However, as the crossing angle 266 is made narrower, an optical coupling loss in the optical-waveguide crossing portion 265 increases more, to pose the problem that a crosstalk characteristic degrades more.
There has been known a cross waveguide which lowers an optical coupling loss in an optical-waveguide crossing portion in such a way that a spot size in the optical-waveguide crossing portion is made larger than a spot size in the optical waveguide outside the optical-waveguide crossing portion by making the width of the optical waveguide of the optical-waveguide crossing portion smaller than the width of the optical waveguide outside the optical-waveguide crossing portion (refer to, for example, Patent Document 2). Even in the structure of such a cross waveguide, however, there has been the problem that the effect of sufficiently lowering the optical coupling loss cannot be attained in a case where a crossing angle is smaller than 30°.
FIG. 42 shows the structural example of a prior-art optical branch circuit. With the expansion of the application fields of optical communication systems, the importance of planar lightwave circuits for branching, multiplexing/demultiplexing and switching signal light(s) has risen more and more. Especially, the optical branch circuit for branching or multiplexing the signal light(s) is indispensable as one of basic constituents in the lightwave circuit.
The optical branch circuit shown in FIG. 42 is configured of an input optical waveguide 371, an optical-waveguide branching portion 372, branched optical waveguides 373a and 373b, and output optical waveguides 374a and 374b (refer to, for example, Non-patent Document 3). The signal light inputted to the input optical waveguide 371 is branched by the optical-waveguide branching portion 372 as well as the branched optical waveguides 373a and 373b, so as to be led to the output optical waveguides 374a and 374b. The optical branch circuit as shown in FIG. 42 is also called the “Y-branch circuit” because of its shape.
As stated above, with the rapid spread of the optical communication systems, the importance of the lightwave circuit for branching an optical signal, switching optical paths, or multiplexing/demultiplexing optical signals/an optical signal every wavelength has increased. For building and providing an optical communication system of high performance, it is indispensable to design and realize a lightwave circuit of high performance.
The lightwave circuit can be designed by combining individual lightwave circuit elements such as a channel optical waveguide, a taper optical waveguide and an optical slab waveguide. However, when such a design method is employed, it is impossible to create a function which cannot be realized by the combination of the prior-art lightwave circuit elements, for example, a spot size converter of very small length. In such a case, the design of the lightwave circuit has heretofore been carried out by employing an optimization technique of cut-and-try type, such as genetic algorithm.
FIG. 43 is a chart representing a design method for a lightwave circuit as is based on a prior-art genetic algorithm (refer to, for example, Non-patent Document 4).
The prior-art algorithm in FIG. 43 includes the step 301 of giving the initial values of refractive index distributions, the step 302 of varying the refractive index distributions in accordance with the genetic algorithm, the step 303 of evaluating the varied refractive index distributions by actually propagating light, the step 304 of selecting favorable refractive index distributions, and the step 305 of judging if the obtained refractive index distributions satisfy desired characteristics. The algorithm first proceeds along the steps 301, 302, 303, 304 and 305, and when the desired characteristics are not obtained at the step 305, the steps 302 through 304 are iterated until the desired characteristics are obtained.
Here, at the step 302 of the prior-art algorithm, the refractive index distribution is altered in accordance with the genetic algorithm. Whether or not the alteration is a change in a better direction has not been known before the light is actually propagated at the step 305.
FIGS. 44A and B show a lightwave circuit (this example is a spot size converter) designed in accordance with the prior-art genetic algorithm (in, for example, Non-patent Document 2).
The lightwave circuit shown in FIGS. 44A and B has a structure in which a core 401 having a constant thickness is embedded in a clad layer 402.
When a light propagation direction is assumed to be along a z-axis, FIG. 44A is a drawing in which a refractive index distribution on a y-axis is observed from the direction of an x-axis, and FIG. 44B is a drawing in which a refractive index distribution on the x-axis is observed from the direction of the y-axis. In the prior-art lightwave circuit shown in FIGS. 44A and B, the optimization of the lightwave circuit is realized in such a way that, as shown in FIG. 44B, the refractive index distribution is divided into segments of constant lengths (3 μm in this example) in the z-axial direction, whereupon the x-axial widths of the respective segments are adjusted in accordance with the genetic algorithm.
Patent Document 1: Japanese Patent Application Laid-open No. 9-297228 (FIG. 7)
Patent Document 2: Japanese Patent Application Laid-open No. 5-60929
Non-patent Document 1: Z. Waissman with one other, “Analysis of Periodically Segmented Waveguide Mode Expanders”, Journal of Lightwave Technology, October 1995, Vol. 13, No. 10 (FIG. 1)
Non-patent Document 2: Michael M. Spuhler with four others, “A Very Short Planar Silica Spot-Size Converter Using a Nonperiodic Segmented Waveguide”, Journal of Lightwave Technology, September 1998, Vol. 16, No. 9 (FIG. 1 and FIG. 2)
Non-patent Document 3: Katsunari Okamoto, “Fundamentals of Optical Waveguides”, 2000 Academic Press (FIGS. 7 and 15)
Non-patent Document 4: B. Plaum with three others, “Optimization of waveguide bends and bent mode converters using a genetic algorithm”, 25th International Conference on Infrared and Millimeter Waves (IRMMW2000), Sep. 12-15, 2000